Process for treating ferrous metals



3,011,907 PROQESS FOR TREATING FERROUS METALS Shipp C. Davis, *Chattanooga, Tenn, and William G. Ray, Atianta, Ga, ssignors to Chas. Pfizer & Co., Inc, New York, N.Y., a corporation of Delaware No Drawing. Fiied Aug. 24, 1960, Ser. No. 51,491 5 Claims. (Cl. 11750) This application is concerned with a new and improved process for porcelainizing metals. More particularly, it is concerned with an improved process for pretreating iron and steel, hereinafter referred to as ferrous metals, to prepare the surface vthereoffor coating with porcelain enamel by a one-coat process.

Patent applications Nos. 768,081 and 831,934, filed October 20, 1958, and August 6, 1959, respectively, both now abandoned, and patent application Serial No. 923, filed January 7, 1960, now Patent No. 2,981,633, describe and claim a novel one-coat process for porcelaiuing ferrous metals comprising the steps of cleaning the ferrous metal, pickling with an aqueous solution of citric acid containing from about 5% to about 20% by Weight of acid at a temperature of from about l60 to about 212 F. for from about 5 to about 30 minutes, depositing nickel on the surface in an amount of from about 0.03 to about 0.15 gram per square foot, treating the object with nickel deposited on the surface thereof With porcelain enamel, drying at from about 250 to about 400 F. and baking same at from about 1300 to about 1550 F. for from about 3 to about 6 minutes.

It has now been discovered that provided certain conditions are met, it is possible to decrease the time that the article to be porcelainized must spend in contact with the pickling solution. This is an extremely important economic consideration in the porcelainizing industry since it makes it possible to enamel cover more areas per unit time. These conditions include the use of critical concentrations of alkali metal thiosulfates or sulfites, preferably sodium thiosulfate or sodium sulfite.

A simplified flow diagram of the process is as follows:

Ferrous Metal Cleaned Rinsed I Pickled, Aqueous Citric Acid Containing Alkali Metal Thiosulfate or Sulfite United rates Patent @l Patented Dec. 5, 1551 It has been found possible in many instances to decrease the pickle time as much as 50% by the addition of one of these reagents to the citric acid bath. Thus, highly purified low carbon steels, i.e., those having less than 0.003% carbon, usually require approximately 20 minutes of pickle time when citric acid alone is used, but by the addition of a reagent of this invention, the pickle time may be decreased to as low as 10 minutes While still obtaining comparable results. With conventional enamel stock or cold rolled steel, the pickle time, when citric acid alone is used, is approximately 5 to 10 minutes. When using the improved process of this invention, the time can be decreased to 3 to 6 minutes.

A number of cleaning methods are available for cleaning the surface to be coated when the process of this invention is to be employed. These include alkali rinses, vapor degreasers and emulsion-type cleaners. Their purpose is to remove greases, oils, soap, dirt and drawing compounds such as soaps, borax and waxes which are app ied to the surface of the metal to provide lubrication during the fabrication process. Typical alkali rinses may contain sodium hydroxide, sodium carbonate, sodium phosphate, sodium silicate or combinations of these. The part to be cleaned is simply immersed in the cleaning compound at about room temperature for a sufficient time to accomplish the desired result. This usually requires from about 10 to about 20 minutes. Vapor degreasers are especially useful for removing hydrocarbon residues from the surface of the metal and in the process, the vapors of a low boiling solvent, for example, a chlorinated hydrocarbon solvent, such as ethylene dichloride, are allowed to condense on the surface of the metal to wash off the residual hydrocarbons. An emulsion-type cleaner may contain a non-ionic surfactant dissolved in naphtha or kerosene and the part to be cleaned is simply rinsed in the solution. Treatment with an emulsion-type cleaner is almost always followed by the alkali rinse described above.

After cleaning, the article is subjected to two aqueous rinses, the first a hot rinse carried out at from about to about F. and the second carried out at from about 55 to about 65 F.

In the next step of the one-coat process, the metal to be porcelainized is pickled with aqueous citric acid at a temperature of from about 160 to about 212 F. In a typical operation, the article, after cleaning by any of the methods described above, is treated with a citric acid solution by either dipping or spraying.

The concentration of citric acid in an aqueous solution suitable for use in this process, varies from about 5% to about 20% by weight, preferably the solution Will contain approximately 8 to 15% by weight of citric acid. Concentrations from 5 to 8% can be employed, but less advantageously since it has been found that these low concentrations require higher temperatures and longer immersion times. It is better economics to use more acid rather than higher temperatures and longer immersion times.

The temperature of the solution is maintained during the treatment at from about 160 to about 212 F., preferably from 190 to 212 F. Higher temperatures within this range will generally be employed if the solution is to be applied by spraying because of the temperature drop inherent in any spraying operation. The treatment is continued from about to about 30 minutes depending upon the gauge of the metal. Since metals other than 12 to 28 gauge are rarely porcelainized, this time range is usually adequate. With gauge steel, the time will be from about 5 to about 20 minutes.

The article to be porcelainized is next placed in a nickel sulfate bath. The purpose of this treatment is to deposit a coating of nickel on the surface of the metal. This deposit enhances the strength of the bond'between the metal and the porcelain enamel. Typically, the nickel sulfate will be prepared in a concentration of approximately /2 ounce to 3 ounces per gallon of water in a wooden tank or a tank lined with lead or brick; The

metal is immersed in the nickel sulfate solution for a period of from about 4 to about 8 minutes at a temperature of from about 160 to 170 F. For optimum results, the pH of the solution is maintained between 3.2 and 3.6, preferably 2.8 to 3.2, by the addition of acid, preferably mineral acids such as sulfuric although at the higher pH, organic acids can be used. When a metal surface is treated in this manner, there is deposited a surface coating of nickel comprising from about 0.03 to about 0.1 gram per square foot. The article is then rinsed with water, preferably by immersion in a water bath at from about 55 to about 65 F.

Following this rinse, the metal is washed with a neutralizing solution, although other solutions may occasionally be used. The preferred solution contains borax and soda ash in a ratio of approximately 2:1. The mixture is dissolved in water at a concentration of approximately /2 ounce per gallon. The temperature is maintained at from about 165 to about 212 F. during the neutralizing operation. The time is from about 2 to about 15 minutes. After the neutralizing operation, the article is rinsed with water at from about 55 to about 212 F. and dried preferably at from about 185 to about 212 F.

The article is now ready for the application of the enamel slip, that is, the porcelain coating. The coating may be any of the various enamel coatings available commercially for cover coats containing the usual refractory materials, opacifiers, flocculating agents, etc. It may also contain coloring agents. The enamel is applied by spraying or dipping in accordance with standard industrial practice. The article is then dried at from about 250 to about 400 F. and baked at from about 1300 to about 150 F. for from about 3 to about 6 minutes. In preferred operations, the article is baked at from about 1350 to about 1500 F. for a period of from about 2 /2 to about 5 minutes.

sodium thiosulfate can be maintained by the addition of 0.1 gram per gallon of this reagent in the pentahydrate form after each 3 square feet of ferrous metal per gallon has been pickled, for example, with a 500 gallon pickle bath, an additional 50 grams should be added after 1500 gram of nickel per square foot of steel.

In the practice of the improved process of this inven-' 7 tion, suflicient alkali metal thiosulfate or sulfite is present in the aqueous citric acid pickle bath described above so as to maintain a concentration offrom about 0.0012 to 0.0030 mole per gallon of alkali metal thiosulfate or 0.0018 to 0.0042 mole of alkali metal sulfite. In preferred operations, the concentration is maintained at from about 0.0018 to about 0.0030 mole per gallon in alkali metal thiosulfate or 0.0030 to 0.0042 rnole per gallon in sodium sulfite.

All calculations are made based on the anhydrous form. With sodium thiosulfate pentahydrate the desired concentrations in grams are from 0.2 to 0.5 gram per gallon. With sodium sulfite they are 0.3 to 0.7 gram per gallon. Other concentrations are readily calculated from the known molecular weights or the salts.

The desired initial concentration of added reagent is readily obtained by weighing out the appropriate quantity square feet of ferrous metal has been pickled. With sodium sulfite, an additional 0.2 gram per square foot is added after each 3 square feet of ferrous metal per gallon has been treated. Alternatively, aliquots of the bath'can be withdrawn and analyzedby known chemical means and the exact quantity required to maintain the desired concentration can be determined and added.

Equivalent quantities'of the other alkali metal salts can be readily calculatedand employed.

Colloidal sulfur is produced when sodium thiosulfate is employed but it is apparent that this does not effect the desired result since no colloidal sulfur is produced when sodium sulfite is employed.

The improvement of this invention, even though the balance of the process is carried out in exact conformance with the earlier procedure, unexpectedly decreases the pickling period to from about 3 to about 15 minutes without sacrifice in appearance or bond in the final product.

It is specifically intended to include within the purview of this invention the pickel bath itself comprising an aqueous solution containing from about 5% to about 20% by weight of citric acid and from about 010012 to about 00030 moleper gallon of alkali metal thiosulfate orfrom about 0.0018 to about 0.0042 mole per gallon of alkali metal sulfite.

, The following examples are given solely for the purpose of illustration and are not to be construed as limitations of this invention, many apparent variations of which are possible without departing from the spirit or scope thereof.

Example I A two-foot by twelve-foot refrigerator bafiie fabricated from an 18 gauge plate of a highly purified steel having a carbon content of 0.003% was cleaned by immersion in an alkaline cleaner and rinsed according to standard practice. It was then immersed in a 10% by weight aqueous solution of citric acid containing 0.0018 mole of sodium thiosulfate per gallon and maintained at a temperature of 200 F. for 10 minutes. It was removed and rinsed with water at room temperature. The baffle was then immersed in a nickel sulfate aqueous solution containing nickel sulfate at a concentration of 1 ounce per gallon for a period of 8 minutes while maintaining the pH between 3.0 and 3.2 by the occasional addition of sulfuric acid. The temperature was maintained at F. This treatment deposited approximately 0.09 The bafile was then rinsed in Water at room temperature and immersed in an aqueous solution containing soda ash and borax in a ratio of 2:1 at a concentration of /2 ounce per gallon. The temperature was maintained at F. for 3 minutes. The baflie was then dried by hot air, coated by spraying with connnercial porcelainenamel and dried at about 250 F. The baffie was then baked at 1400 F. for 4 minutes. The bond and appearance was excellent.

It was found that in order to obtain comparable bond and appearance under identical conditions except that the citric acid bath did not contain added sodium thiosulfate, it was necessary to maintain the bathe in the pickle bath for 20 minutes.

Similar results are obtained using the thiosulfate salts of potassium, lithium and cesium.

Example 11 A three-foot by four-foot side panel of a commercial Washing machine fabricated with conventional cold rolled steel and having a carbon content of approximately 0.03% was cleaned by allowing the vapors of ethylene dichloride to condense on its surface. It was then immersed in a 20% by weight aqueous solution of citric acid containing 0.0012 mole of sodium thiosulfate per gallon and maintained at a temperature of 160 F. for five minutes. It was removed and rinsed with Water at room temperature. The side panel was then immersed in a nickel sulfate aqueous solution containing nickel sulfate at a concentration of /2 ounce per gallon for a period of eight minutes while maintaining the pH between 3.2 and 3.6 by the occasional addition of solid boric acid. The temperature was maintained between 160 and 170 F. The article was then immersed in an aqueous solution containing borax and soda: ash in a ratio of 2:1 at a concentration of /2 ounce per gallon. The temperature was maintm'ned at approximately 165 F. forfifteen minutes. The article was then rinsed with water at 185 F. and dried. It was then coated by immersion with commercial porcelain enamel, dried at 400 F. and baked at 1550 F. for three minutes. The bond and-appearance were excellent. 7

It was found that in order to obtain comparable bond and appearance under identical conditions except that the citric acid bath did not contain added sodium thiosulfate, it was necessary to maintain the panel in the pickle bath for ten minutes.

Similar results are obtained using the thiosulfate salts of potassium, lithium and cesium.

Example 111 A three-inch by six-inch piece of twelve gauge highly purified steel having a carbon content of 0.003% was cleaned by immersing in aqueous solid hydroxide and rinsed according to standard practice. it was then immersed in 15% by weight aqueous solution of citric acid containing 0.0030 mole per gallon of sodium thiosulfate at 160 F. for fifteen minutes. It was removed and rinsed with water at room temperature. The article was then immersed in a nickel sulfate bath containing nickel sulfate at a concentration of two ounces per gallon for a period of four minutes, while maintaining the pH between 3.2 to 3.6 by the occasional addition of solid citric acid. The temperature was maintained between 160 and 170 Ff This treatment deposited approximately 0.05 to 0.07

gram of nickel per square foot of surface. The article was then immersed in an aqueous solution containing borax and soda ash in a ratio of 2:1 at a concentration of /2 ounce per gallon. The temperature was maintained at approximately 212 F. for two minutes. The article was then rinsed with water at 212 F. and dried. It was then coated by spraying with commercial porcelain enamel, dried at about 250 F. and baked at 1300 F. for six minutes. The bond and appearance were commercially acceptable.

It was found that in order to obtain comparable bond and appearance under identical conditions except that the citric acid bath did not contain added sodium thiosulfate, it was necessary to maintain the article in the pickle bath for thirty minutes.

Similar results are obtained using the thiosulfate salts of potassium, lithium and cesium.

Example IV A three-inch by six-inch piece of sixteen gauge mild steel was cleaned with an emulsion type cleaner followed by an immersion in 30% aqueous sodium hydroxide. It was then immersed in a 15% by weight aqueous solution of citric acid containing 0.0018 mole per gallon of sodium sulfite at 212 F. for three minutes. It was removed and rinsed with water at room temperature. The article was then immersed in a nickel sulfate bath containing nickel sulfate at a concentration of four ounces per gallon for a period of four minutes while maintaining the pH between 3.2 and 3.6 between occasional addition of solid citric acid. The temperature was maintained between and 170 F. It was then immersed in an aqueous solution containing borax and soda ash in a ratio 2:1 at a concentration of /2 ounce per gallon. The temperature was maintained at approximately 212 F. for two minutes. The article was then rinsed with water at room temperature and dried. It was then coated by spraying with commercial porcelain enamel, dried at about 350 F. and baked at 1450 F. for four minutes.

It was found that in order to obtain comparable bond and appearance under identical conditions except that the citric acid bath did not contain added sodium sulfite and was necessary to maintain the article in the pickle bath for six minutes.

Similar results are obtained using the sulfite salts o potassium, lithium and cesium.

Example V A three-inch by six-inch piece of twenty-eight gauge steel containing 0.03% carbon was cleaned by allowing the vapors of ethylene dichloride to condense on its surface. It was then immersed in a 20% by weight of aqueous solution of citric acid containing 0.00042 mole per gallon of sodium sulfite at 160 F. for five minutes. It was removed and rinsed with water at room temperature. The article was then immersed in a nickel sulfate bath containing nickel sulfate with a concentration of /2 ounce per gallon for a period of eight minutes while maintaining the pH between 2.8 and 3.2 by the occasional addition of solid boric acid. The temperature was maintained between 160 and 170 F. The article was then immersed in an aqueous solution containing borax and soda ash in the ration 2:1 at a concentration of /2 ounce per gallon. The temperature was maintained at approximately F. for fifteen minutes. The article was then rinsed with water at 185 F. and dried. it was then coated by immersing with commercial porcelain enamel and dried at about 400 F. The article was then baked at 1550 F. for three minutes. The bond and appearance were of commercial grade.

Example VI A three-inch by six-inch piece of highly purified steel containing 0.003% carbon was cleaned by immersing in aqueous sodium hydroxide and rinsed according to standard practice. It was then immersed in a 15% aqueous solution of citric acid containing 0.00020 gram of sodium thiosulfate per gallon at 160 F. for ten minutes. It was removed and rinsed with water at room temperature. The article was then immersed in a nickel sulfate bath containing nickel sulfate in a concentration of two ounces per gallon for a period of four minutes while maintaining the pH between 2.8 and 3.2 by the occasional addition 01' solid citric acid. The temperature was maintained between 160 and F. This treatment deposited approximately 0.05 to 0.07 gram of nickel per square foot of surface. It was then immersed in an aqueous solution containing borax and soda ash in a ration of 2:1 at a concentration of /2 ounce per gallon. The temperature was maintained at approximately 212 F. for two minutes. The article was then rinsed with water at 212 F. and dried. it was coated by spraying with a commercial porcelain enamel, dried at 250 Fpand baked at 1300 F. for six minutes. The bond and appearance were of commercial grade.

It was found that in order to obtain comparable bond and appearance under identical conditions except that I the citric acid bath did not contain added sodium thicsulfate, it was necessary to maintainthe article in the pickle bath for twice as long. 7

Similar results are obtained using the thiosulfate salts, potassium, lithium and cesium. i

What is claimed is:

1. A process which comprises pickling clean ferrous metal objects with an aqueous solution of citric acid containing from about to about 20% by Weight of acid together with a reagent selected from the group consisting of alkali metal thiosulfates and alkali metal sulfites, the concentration of alkali metal thiosulfate being from about 0.0012 to about 0.0030 mole per gallon and the concentration of alkali metal sulfite being from about 0.0018 to about 0.0042 moleper gallon at a temperature of from about 160 to about 212 F. for from about 3 to about 15 minutes, depositing on the surface thereof from about 0.03 to about 0.15 gm. of nickel per sq. ft., applying a porcelain enamel coating, drying at from about 250 to about 400 F. and baking at from about 1300"- to about 1550 F. for from about 3 to about 6 minutes.

2. Process as in claim 1 wherein the alkali metal thiosulfate is sodium thiosulfate.

3. A process as in claim 1 wherein the alkali metal sulfite is sodium sulfite.

4. In a process for the porcelain enamelingof ferrous metal objects comprising the steps of cleaning the ferrous metal, pickling with an aqueous solution of citric acid containing from about 5% to about 20% by weight of acid-at a temperature of from about 160 to about 212 F., depositing nickel on the surface in an amount .of from about 0.03 to about 0.15 gm.. per sq. ft., treat ing the object with nickel deposited on the surface thereof with porcelain enamel, drying at from about 250 to about 400 F. and baking at from about 1300 to about 1550 F. for from about 3 to about '6 minutes, the improvement which comprises adding a sufiicient amount of a reagent selected from the group consisting of alkali metal thiosulfates and alkali metal sulfites to maintain a concentration of from about 0.0012 to about 0.0030 mole per gallon in the case of alkali metal thiosnlfa'tes and from about 0.0018 to about 0.0042 mole per gallon in the case of alkali metal sulfites and pickling for from about 3 to about 15 minutes.

5. A pickling bath comprising an aqueous solution containing from about 5%[to about 20% by Weight of citric acid together with a reagent selected from the group consisting of alkali metal thiosulfates and alkali metal sulfites, the concentration of alkali metal salts being from about 0.0012 to about 0.0030 mole per gallon in the case of alkali metal thiosulfate s and from about 0.0018; to about 0.0042 mole per gallonin the case of alkali metal sulfites.

References Cited in the file of this patent UNITED STATES PATENTS 865,700 Hernsheim et a1 Sept. 10, 1907 1,334,092 Harmelinget al. Nov. 16, 1920 2,768,904 Chester Oct. 30, 1956 elm. 

1. A PROCESS WHICH COMPRISES PICKLING CLEAN FERROUS METAL OBJECTS WITH AN AQUEOUS SOLUTION OF CITRIC ACID CONTAINING FROM ABOUT 5% TO ABOUT 20% BY WEIGHT OF ACID TOGETHER WITH A REAGENT SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL THIOSULFATES AND ALKALI METAL SULFITES, THE CONCENTRATION OF ALKALI METAL THIOSULFATE BEING FROM ABOUT 0.0012 TO ABOUT 0.0030 MOLE PER GALLON AND THE CONCENTRATION OF ALKALI METAL SULFITE BEING FROM ABOUT 0.0018 TO ABOUT 0.0042 MOLE PER GALLON AT A TEMPERATURE OF FROM ABOUT 160* TO ABOUT 212*F. FRO FROM ABOUT 3 TO ABOUT 15 MINUTES, DEPOSITING ON THE SURFACES THEREOF FROM ABOUT 0.03 TO ABOUT 0.15 GM OF NICKLE PER SQ. FT., APPLYING A PORCELAIN ENAMEL COATING, DRYING AT FROM ABOUT 250* TO ABOUT 400*F. AND BAKING AT FROM ABOUT 1300* TO ABOUT 1550*F. FOR FROM ABOUT 3 TO ABOUT 6 MINUTES. 